A device for measuring particle size distributions is known from GB 2 203 542 A. The measurement of particle sizes of a particle stream of low concentration is carried out by forward light scattering on the basis of recorded Fraunhofer diffraction patterns. A high-intensity light source, e.g., a pulsed gallium arsenide laser, is needed for the process. Due to the high-intensity light source, a blocker is used to block the non-scattered light or the light scattered in the forward direction.
This special device shall make it possible to detect very small particles of sizes smaller than 1 μm in diameter even at low concentrations.
Such particles have sizes on the order of magnitude of the wavelength of the light being used. If the particles become markedly larger than the wavelength of the light, an analysis of the diffraction pattern may, however, lead to misinterpretations, especially if the particle has a shape markedly differing from the spherical shape. The diffraction thus becomes more complex, which cannot now be unambiguously discriminated, among other things, from an intensity distribution from a superposition of several separate particles. In addition, the problem is that a diffraction measurement is unable to distinguish agglomerated small particles from an individual, larger particle.
Since the process described in GB 2 203 542 A is not well suited for large particles, the problem is that very broad size distributions must be detected correctly.